A Center’s primary function is to control and separate
air traffic within a designated airspace, which may cover
more than 100,000 square miles, may span several
states, and extends from the base of the underlying controlled
airspace up to Flight Level (FL) 600. There are
21 Centers located throughout the U.S., each of which is
divided into sectors. Controllers assigned to these sectors,
which range from 50 to over 200 miles wide, guide
aircraft toward their intended destination by way of vectors
and/or airway assignment, routing aircraft around
weather and other traffic. Centers employ 300 to 700
controllers, with more than 150 on duty during peak
hours at the busier facilities. A typical flight by a commercial
airliner is handled mostly by the Centers.

TERMINAL RADAR APPROACH CONTROL

Terminal Radar Approach Control (TRACON) controllers
work in dimly lit radar rooms located within
the control tower complex or in a separate building
located on or near the airport it serves. [Figure 1-9]
Using radarscopes, these controllers typically work
an area of airspace with a 50-mile radius and up to an
altitude of 17,000 feet. This airspace is configured to
provide service to a primary airport, but may include
other airports that are within 50 miles of the radar
service area. Aircraft within this area are provided
vectors to airports, around terrain, and weather, as
well as separation from other aircraft. Controllers in
TRACONs determine the arrival sequence for the control
tower’s designated airspace.

CONTROL TOWER

Controllers in this type of facility manage aircraft operations
on the ground and within specified airspace
around an airport. The number of controllers in the
tower varies with the size of the airport. Small general
aviation airports typically have three or four controllers,
while larger international airports can have up
to fifteen controllers talking to aircraft, processing
flight plans, and coordinating air traffic flow. Tower
controllers manage the ground movement of aircraft
around the airport and ensure appropriate spacing
between aircraft taking off and landing. In addition, it
is the responsibility of the control tower to determine
the landing sequence between aircraft under its control.
Tower controllers issue a variety of instructions to pilots, from how to enter a pattern for landing to how
to depart the airport for their destination.

Pilot Briefers at flight service stations render preflight,
in-flight, and emergency assistance to all pilots on
request. They give information about actual weather
conditions and forecasts for airports and flight paths,
relay air traffic control instructions between controllers
and pilots, assist pilots in emergency situations, and
initiate searches for missing or overdue aircraft.
FSSs/AFSSs provide information to all airspace users,
including the military. In October 2005, operation of
all FSSs/AFSSs, except those in Alaska, was turned
over to the Lockheed Martin Corporation. In the
months after the transition, 38 existing AFSSs are
slated to close, leaving 17 “Legacy” stations and 3
“Hub” stations. Services to pilots are expected to be
equal to or better than prior to the change, and the
contract is expected to save the government about
$2.2 billion over ten years.

FLIGHT PLANS

Prior to flying in controlled airspace under IFR conditions
or in Class A airspace, pilots are required to file a
flight plan. IFR (as well as VFR) flight plans provide
air traffic center computers with accurate and precise
routes required for flight data processing (FDP1). The
computer knows every route (published and unpub-lished) and NAVAID, most intersections, and all airports,
and can only process a flight plan if the proposed
routes and fixes connect properly. Center computers
also recognize preferred routes and know that forecast
or real-time weather may change arrival routes.
Centers and TRACONs now have a computer graphic
that can show every aircraft on a flight plan in the U.S.
as to its flight plan information and present position.
Despite their sophistication, center computers do not
overlap in coverage or information with other Centers,
so that flight requests not honored in one must be
repeated in the next.